By DONALD G. MCNEIL JR

Published: July 29, 2008

For a mother infected with the virus that causes AIDS, breast-feeding presents a life-and-death dilemma. Using formula protects her child from H.I.V. But mixed with dirty water, it increases the chances that the child will die of diarrhea or malnutrition. In poor countries, most mothers still breast-feed, because it is traditional, because formula is expensive -- and because using formula announces to local busybodies that a mother is probably infected.

As a result, at least 150,000 babies are infected by their mothers each year through breast-feeding, experts estimate.

In trying to prevent that, AIDS researchers have stumbled into their own terrible dilemma. One dose of nevirapine, an inexpensive antiretroviral drug, can prevent mother-to-child transmission. But a single dose doesn't always work. And nevirapine can be dangerous; some infants develop liver failure, rashes or low white blood cell counts. Also, resistance to it develops quickly, meaning that a child who gets nevirapine and becomes infected anyway will be unable to take triple-therapy cocktails containing it or any related drug.

Last week, the journal The Lancet described trials in which 2,000 babies in Ethiopia, India and Uganda were given six weeks of nevirapine while breast-feeding. The trials, which found that six weeks was not clearly protective, only added to the controversy -- a Lancet editorial called some results ''head scratchers.''

The principal authors suggested that infants might therefore need more nevirapine. But some of the Indian investigators dissented publicly, saying in a letter to The Lancet that the drug was too risky and that safer formula-feeding should be encouraged instead.